Technical Field
The present disclosure relates to a base material-integrated nanocrystalline metal oxide composite-containing catalyst, a method for manufacturing the same, and a catalyst component.
Description of the Related Art
Generally, a catalyst means a substance which changes the reaction velocity of a substance system providing a chemical reaction without chemically changing itself. Examples of a catalyst component using a catalyst include a catalytic converter used in order to clean exhaust gases discharged from an automobile engine.
Usually, the catalytic converter mainly includes a case made of a metal material such as stainless steel, a carrier stored in the case, having a cell structure, and generally made of a porous ceramic material, and a holding material mat disposed so as to cover the periphery of the carrier. The carrier is manufactured by a so-called wash coat treatment for immersion into an aqueous solution in which catalyst particles or ceramic particles made of alumina (Al2O3), silica (SiO2) and the like are dispersed, followed by pulling up and drying or firing (sintering). The carrier has a structure where catalyst particles, or ceramic particles such as alumina particles are supported on the wall surface of a cell.
It is general to use, as catalyst particles, precious metal catalysts such as platinum (Pt), rhodium (Rh), and palladium (Pd) and the like. However, since the precious metal catalyst is expensive, and has a resource depletion problem, recently, attempts have been positively made to reduce the amount of the precious metal catalyst to be used.
As means for reducing the amount of the precious metal catalyst to be used, for example, by making catalyst particles finer to nanoparticles having a nanometer scale particle diameter of less than 1 μm, it is useful to increase the area (surface area) ratio of a catalyst surface (active surface) producing a catalyst reaction, or to substitute an inexpensive transition metal or oxide thereof for a part of the precious metal catalyst.
However, when the precious metal catalyst is merely made finer to the nanoparticles without controlling the surface quality of the catalyst, and the nanoparticles have a smooth spherical surface, the surface area of the catalyst particle cannot be effectively increased. A part of the surface of the catalyst particle (for example, a lower half part of the surface of the catalyst particle) is usually supported on the surface of the carrier, or in a state of being buried in the surface of an alumina particle supported on the carrier, and becomes a supported surface which cannot produce a function as an active surface. The supported surface certainly exists at a certain level of area ratio (for example, about 10 to 50% of the total surface area of the catalyst particle), which causes the following problem: the surface area of the active surface of the actual catalyst particle cannot be desirably increased.
For this reason, the surface area of the supported surface of the catalyst particle is preferably narrowed as much as possible from the viewpoint of increasing the number of active points. When the supported surface is excessively narrowed, a support strength is insufficient, which causes the catalyst particle to readily detach from the surface of the carrier, as a result of which sufficient catalyst activity is not obtained. This requires a catalyst which provides a support strength preventing the detachment of the catalyst particle from the carrier, and has a surface quality capable of securing the area (surface area) of a sufficient active surface.
For example, Japanese Laid-Open Patent Publication No. 2013-240756 (JP 2013-240756A) discloses that a single nanocrystalline board-accumulative catalyst (nanoflower) is developed, in which single nanocrystalline boards each having a specific surface of a specific single crystal regarded as one surface are accumulated without facing and contacting the catalyst active surfaces between adjacent single nanocrystalline boards. JP 2013-240756A also discloses that: by using the catalyst, a space (void portion) is secured in front of the catalyst active surface without facing and contacting the catalyst active surfaces even during thermal aggregation; deterioration in the catalyst activity caused by the thermal aggregation can be suppressed; and catalyst activity can be improved. JP 2013-240756A further discloses that by using a CuO single nanocrystalline board which has a catalyst active surface as a (001) plane and is a transition metal oxide, as the single nanocrystalline board, the material cost of the catalyst can be reduced.
However, the single nanocrystalline board-accumulative catalyst described in JP 2013-240756A has a structure where the adjacent single nanocrystalline boards merely accumulate. This causes the following problems: a binding strength between the adjacent single nanocrystalline boards of the single nanocrystalline board-accumulative catalyst is weak; the single nanocrystalline board forming the single nanocrystalline board-accumulative catalyst readily detaches even under small vibration or impact; and the single nanocrystalline board-accumulative catalyst readily breaks without remaining the shape of the single nanocrystalline board-accumulative catalyst itself.
A method for manufacturing the single nanocrystalline board-accumulative catalyst described in JP 2013-240756A is a method for manufacturing a single nanocrystalline board-accumulative catalyst by a hydrothermal reaction in a sealing state where a mixed solution obtained by mixing CuCl2 and urea with an aqueous solution containing an organic solvent such as ethylene glycol of 50 mol % or less is disposed in a pressure vessel. The method requires a long reaction time of 10 hours or more, and is a manufacturing method in a sealing state using the pressure vessel. Therefore, the method has the following industrial production problems: it is difficult to manufacture the single nanocrystalline board-accumulative catalyst; and only a small amount of the single nanocrystalline board-accumulative catalyst can be manufactured per manufacture, which causes poor mass productivity.